1. Field of the Invention
This invention relates to a semiconductor pressure sensor (to be abbreviated to a "pressure sensor" hereinafter), and, more particularly, to a pressure sensor for precisely measuring pressure by reducing residual stress resulting from the manufacture of the pressure sensor.
2. Description of the Related Art
FIG. 12 is a cross sectional view which illustrates a conventional pressure sensor disclosed in, for example, U.S. Pat. No. 4,655,088. FIGS. 13 and 14, respectively, cross sectional and plan views which illustrate the outer package of the pressure sensor shown in FIG. 12. Referring to the drawings, a pressure sensor chip is secured, by a die bonding material 6, to an outer package 10 called an "integrated mold" made of, for example, epoxy resin. The outer package 10 has an opening 13 formed therein and leads 8 provided therein. The pressure sensor chip 1 and the lead 8 are connected to each other by wires 9, and the upper portion of the pressure sensor chip 1 is covered with a protecting resin 14. A dust-protecting disc 12 having an opening 11 formed therein is disposed in the upper portion of the pressure sensor chip 1.
The conventional pressure sensor arranged as described above is manufactured in such a manner that it is integrally molded together with the leads 8 to form the outer package 10. Subsequently, the pressure sensor chip 1 is die bonded by the die bonding material 6, and the pressure sensor chip 1 and the leads 8 are wire bonded to the wires 9. Then, the protecting resin 14 is applied to the surface of the pressure sensor chip 1, and a disc 11 is adhered to the outer package 10 so that the pressure sensor is manufactured. As described above, the pressure sensor chip 1 is, by the die bonding material 6, adhered to the outer package 10 which has been molded integrally with the leads 8. Therefore, the wires 9 and the pressure sensor chip 1 are not integrally molded with the outer package 10, but only the leads 8 and the outer package 10 are integrally molded.
In order to improve the characteristics of the thus constituted pressure sensor by reducing the residual stress, it is most effective to arrange the component to be made of a material having a linear expansion coefficient which approximates the linear expansion coefficient of silicon, the material for the pressure sensor chip 1. That is, the accuracy of the pressure sensor is determined by the physical properties of the material employed. The reason for this lies in that the residual stress generated during manufacturing acts on the resistor of the pressure sensor, causing the accuracy in the measurement of the pressure to be reduced.
Hitherto, the offset drift linearity (the temperature change linearity of the offset voltage) and the span drift linearity (the output voltage linearity when pressure is applied) have been substantially determined by the structure and the material of the pressure sensor. Therefore, the material and the structure must be changed in order to obtain satisfactory characteristics. For example, the pressure sensor shown in FIG. 12 suffers from poor characteristics, and the obtainable accuracy therefore limits the fields in which it can be used. The reason for this lies in that the residual stress generated after the components have been assembled is too large because the difference between the linear expansion coefficient, 3.5.times.10.sup.-6 (1/.degree.C.) of silicon which forms the pressure sensor chip 1 and the linear expansion coefficient, 33.times.10.sup.-6 (1/.degree.C.), of, the epoxy resin forming the outer package 10 is too large since the outer package 10 is directly die bonded to the pressure sensor 1.
Accordingly, in order to produce an accurate pressure sensor, a silicon pedestal made of the same material as the material for the silicon chip or, of example, a pyrex glass pedestal having substantially the same linear expansion coefficient as that of the silicon chip is integrally molded with the silicon chip, the thus molded pedestal being die bonded to the lead frame or the stem so that the residual stress is reduced. At this time, the more the thickness of the pedestal, the greater significant is the reduction in the residual stress. However, if the pedestal has the large thickness, the temperature at the wire bonding pad portion of the pressure sensor chip cannot easily be raised at the time of wire bonding. As a result, the temperature of a heater for heating the wire bonding pad portion must be raised. However, the temperature cannot be raised to a temperature higher than the durable temperature of the molding resin. Accordingly, the wire bonding must be performed by the application of ultrasonic oscillations and force. However, if the level of the ultrasonic oscillations is raised, another problem arises in that the plating must have a large thickness on the leads.